Separation media, such as chromatography media and filtration media, are often associated with non-satisfactory properties to some end. Important factors in this field are e.g. the mass transport properties of the media, the flow properties thereof when used in chromatographic columns or as membranes, cumbersome and non-reliable methods of preparation etc. Hence, there is an ongoing development to seek improvements in this field.
Reeder, et al. (An approach to hierarchically structured porous zirconia aggregates. Reeder et al., Dep. Chem. Eng., Univ. Minnesota, Minneapolis, Minn., USA. Journal of Colloid and Interface Science (1996), 184(1), 328-330) disclose an approach to aggregate colloids into hierarchically structured spherical particles. Successive aggregation steps are used to assemble an inorganic particle that is self-similar on 2 size scales and is permeated by an ordered pore network with a bidisperse size distribution. The structure of the micro- and macro-pore networks as well as the mechanical integrity of the structure can be controlled by varying sintering conditions. One drawback of the disclosed aggregation is that the successive steps required will render the preparation thereof time-consuming and costly. This approach is also limited to inorganic particles that can be sintered.
U.S. Pat. No. 4,070,286 disclose a powder of discrete, macroporous, microspheroids each composed of a plurality of large colloidal particles joined and cemented together at their points of contact by non-porous amorphous silica. The cementing is done with a partial sintering process, which is only possible for inorganic materials. Also, the coacervation process described makes it difficult to control the sphericity of the aggregates.
O. D. Velev, K. Furusawa, K. Nagayama, have in Langmuir, 12, 2374-2384 (1996), Langmuir, 12, 2385-2391 (1996) and Langmuir, 13, 1856-1859 (1997) described assembly of latex particles into spherical aggregates by using emulsion droplets as colloid templates. The aqueous latex solution is dispersed by a homogenizer inside anhydrous octanol medium. The removal of water occurs by partitioning into the octanol phase. This means that it starts as soon as the phases are brought into contact with each other, leaving no time for controlled emulsification to produce the required droplet size. The amount of water that can be removed is also limited by the equilibrium water solubility in the amount of octanol present.
U.S. Pat. No. 6,599,620 relates to a cellulosic particle body comprising interconnected cellulosic small particles with small interparticle spaces and to method for production thereof comprising dispersing small cellulosic particles in an alkaline medium and contacting the resulting suspension with a coagulating solution. The precipitation of the binder in the coagulating solution proceeds in an uncontrolled fashion and it is difficult to obtain desired open pore structures. The spraying processes involved also make it difficult to obtain aggregate sizes suitable for chromatography and in sufficient yield.
U.S. Pat. No. 3,782,075 discloses a packing material for chromatography columns, prepared from a powder of uniform-sized porous microspheres composed of a plurality of interconnected colloidal oxide particles. The size of the pores is controlled by the size of the colloidal particles used to form the microspheres and the surface area of the microsphere is controlled by the amount of sintering used to impart strength to the particles. The primary particles are joined together by sintering, which limits the applicability to inorganic particles.
EP 0 442 977 relates to a chromatography method using a matrix comprising interconnected first and second throughpore sets. The members of the first throughpore set have a greater mean diameter than the members of the second throughpore set. The second throughpore set is in fluid communication with solute interactive regions which interact reversibly with solutes to effect chromatographic separation thereof. The method of preparing the matrix appears unclear and not fully reproducible which means that not all batches will show effects of a hierarchical structure. Furthermore, it may be difficult to control sizes and connect throughpores with this method.
In spite of all prior art within this technical field, there is still a need in this field of improved methods to prepare separation media as well as of novel separation media.
Definitions
The term “hierarchical” or “fractal” is used herein to describe a porous structure with large pores open to the exterior. On the walls of these large pores, a system of smaller pores opens. Optionally, a further system of even smaller pores may open on the walls of these pores etc.
The term “separation medium” is used herein in a broad sense to include any material that is useful as the stationary phase in a separation method, such as a chromatographic process or a filtration. The medium can be used as such or combined with another material, such as a rigid support in a filtration. Further, a “separation medium” as used herein will include both materials that are directly useful for adsorption or sieving and such materials having additional adsorbing groups, known as ligands, coupled thereon.